1. Field of the Invention
This invention generally relates to a power supply unit and a printing apparatus. More particularly, this invention relates to a main power supply unit and a supplemental power supply unit for a printing apparatus.
2. Description of the Background Art
Typical printing apparatuses of this field are disclosed, for instance, Japanese Open-Laid Patent 2004-236492 and Japanese Open-Laid Patent 2005-221674. These apparatuses have both a main power supply unit and a supplemental power supply unit, wherein the printing apparatuses switch a source of DC power provided to a part of the printing apparatus from the main power supply unit to the supplemental power supply unit when the total amount of DC power consumption exceeds the rated power of the outlet. However, an output voltage of the main power supply unit and an output voltage of the supplemental power supply unit have slightly different voltages.
This voltage difference can cause a malfunction of motor movements (e.g., an unexpected motor stop or uneven rotation of a motor). Further, for a color printing apparatus, the voltage difference can result in rotations between motors which leads to a color registration difference.
Other problems associated with unsynchronized switching of power between two power supply units is that it requires a grouping of DC loads in the printing apparatus. One group is always supplying power from the main power supply unit and the other group is supplying power from both the main power supply unit and the supplemental power supply unit. This grouping often restricts the design of the printing apparatus.